The present invention relates to the field of extrusion die engineering, and in this field to a method for manufacturing an extrusion die. A suitable substance for extrusion, typically an aluminium alloy, is forced under high pressure through an aperture that is delimited by an extrusion die, and during this extrusion process and while passing through the die, the substance has a ductile, highly viscous consistency such that it may be formed into a suitable, even complex shape configuration, depending on the design of the extrusion die.
Due to the special circumstances associated with extrusion technology, most importantly the fact that the ductile extrusion substance advances continuously over the (stationary) functional surface of the tool under high pressure and at high temperature, particular requirements must be imposed on both the design of the tool and the material from which the tool is constructed. Firstly, the functional surface that constitutes the contact area with the extrusion substance must be exceptionally resistant to wear, a characteristic that is typically achieved by applying a coating that suitably increases the hardness of the surface, or by subjecting the substance to a surface hardening process (nitriding or the like). Secondly, the particular conditions of extrusion and the particular geometries (such as long tabs or thin crosspieces) of the extrusion shapes that are to be produced also demand that the tool possess a certain resistance, with the consequence that the use of very hard (but brittle) substances, hard metals or high speed steels for example, which would otherwise be conceivable, often proves impossible. The high operating temperatures in extrusion dies, typically between 550° C. and 640° C., also impose greater requirements on the steels used for the die in terms of resistance to prolonged heat, which in turn means that cold work steels, for example, lend themselves less readily to use in an extrusion die.
From the prior art, it is known and has proven a reliable way to increase the wear resistance (and thus also the service life) of an extrusion die, if a coating is applied in a CVD process. For example, EP 1 011 885 B1 by the same Applicant discloses a method for coating an extrusion die by means of high-temperature CVD, in which a metal phase is deposited (in otherwise known manner) on the surface of the suitably prepared and shaped die; a preferred processing temperature for such a process is above 950° C., in order to ensure optimum reactivity of the gases used for gas deposition.
From WO 2012/175147 A from the same Applicant, it is further known to coat an extrusion die by means of chemical gas phase deposition (CVD) in the “mid-temperature” range. Such a process, particularly with greater concentrations of carbon in the deposition gas, lends increased resilience and elasticity to the functional surface of the extrusion die (when it is coated in this way), which is advantageous for extrusion technology for the reasons stated in the introduction.
Finally, it is also known from the prior art to require the use of materials consisting of hard metals or non-ferrous alloys to produce an extrusion die. However, these materials are very brittle, and it is very difficult to shape them when making dies, with the result that these technologies have only found limited practical use.
A further drawback is that these materials are expensive, and it is seldom possible to create extrusion dies economically with them.